The obvious one is to change your scalastyle config so that the warnings you want to cause build failure are errors. That's really what the scalastyle config is for. If you want something to be treated as an error, call it one! <check level="error" ...> will give you the least amount of headache.

Otherwise, the only simple way to promote a warning to an error in sbt is to use the -Xfatal-warnings flag:

scalacOptions ++= Seq("-Xfatal-warnings")

But that will convert all warnings in your project to errors, scalastyle or not.